This invention relates to the defluorination of phosphate rock and, more particularly, to the preparation of a defluorinated phosphate animal feed supplement from phosphate rock.
Defluorinated phosphate has found wide acceptance as a supplement for animal feed. Specifications for "defluorinated phosphate" or "feed grade tricalcium phosphate" which require a minimum of 18 weight percent phosphorus (P) content and a maximum of 0.18 weight percent fluorine (F) content have become standard within the industry. The weight ratio of phosphorus to fluorine, P/F weight ratio, is specified as greater than 100. This P/F weight ratio specification is a requirement for sale of defluorinated phosphate as an animal feed additive in many states of the United States.
A variety of processes have been suggested for thermal defluorination of phosphate rock to prepare defluorinated phosphate. Addition of one or more reagents in minor proportion to phosphate rock and heating to relatively high temperatures in the presence of water vapor are common features of processes described for thermal defluorination. The processes may be differentiated as to whether the product is in the molten or solid state as it leaves the furnace or calciner.
A disadvantage of processes which heat phosphate rock mixed with a reagent or reagents above the melting point is high unit energy consumption due to the high temperatures required. Unit energy consumption is the quantity of energy input per quantity of product output. A further disadvantage is the very corrosive effect of molten phosphate materials on furnace refractories. One such process is described by Hignett and Hubbuck, Ind. Eng. Chem., 38., 1208, 1946.
Processes which discharge the product from the calciner as a solid operate at lower temperatures than those that discharge the defluorinated product from the furnace as a melt. Because the temperature required for defluorination of phosphate rock is near the fusion point, objectionable sintering or fusion may occur if the temperature in the calciner inadvertantly rises during calcination. Reagents such as SiO.sub.2 or Na.sub.2 O which aid defluorination also lower the melting point.
To allow defluorination without objectionable sintering and fusion, Hollingsworth, in U.S. Pat. No. 2,839,377 suggests control of the molar ratio of: ##EQU1## between the values 1.3 and 2.8 with addition of phosphoric acid and a sodium salt to allow calcination at a temperature of at least 2600.degree. F. without substantial fusion. As a further improvement, Hollingsworth, in U.S. Pat. No. 2,995,436, describes forming porous nodules from a mixture containing phosphate rock having 2% to 6% silica content, 5% to 9% Na.sub.2 O from soda ash and 7% to 11% P.sub.2 O.sub.5 from phosphoric acid (based on the dry weight of the mixture) to allow complete defluorination by calcination without fusion at temperatures within the range 2500.degree. to 2700.degree. F. Hollingsworth describes additional improvement of the granulation technique for phosphate rock, phosphoric acid and soda ash in U.S. Pat. No. 3,189,433.
Amin, U.S. Pat. No. 3,852,493 suggests a modification of the composition and procedure of U.S. Pat. No. 2,995,436 to allow use of phosphate rock analyzing 74/73 BPL (BPL=% bone phosphate of lime or Ca.sub.3 (PO.sub.4).sub.2 equivalent to P.sub.2 O.sub.5 content of the rock) rather than phosphate rock analyzing over 75 BPL.
Hollingsworth and Snyder describe in U.S. Pat. No. 3,364,008 a technique of thin film defluorination in a fluid bed calciner. Non-agglomerated phosphate feed solids finer than minus 35 mesh are injected into a fluid bed of larger seed particles maintained at a temperature sufficient to cause the particles to stick together and as a consequence of physical contact between the feed particles and other particles in the fluid bed to agglomerate and thereby increase in size. The agglomerated particles are subjected to calcination and recovered as a defluorinated and agglomerated phosphate rock having a particle size appreciably larger than the non-agglomerated feed particles. Partial reaction may be effected before introduction of the non-agglomerated feed into a fluid bed for agglomeration and calcination. Agglomeration may also be effected in a fluid bed maintained at above 1000.degree. F. before introduction into a second fluid bed for calcination.
Larson et al, in U.S. Pat. No. 4,101,636 describe the addition of a lime compound to phosphoric acid before the phosphoric acid is mixed with phosphate rock containing more than 6% acid insoluble impurities together with a sodium compound such as sodium carbonate before granulation followed by calcination in a fluid bed calciner at a temperature within the range of from about 980.degree. to about 1350.degree. C.
One disadvantage of the processes heretofore proposed for fluid bed calcination is the use of excessive quantities of energy in calcination. The aforementioned processes use only one state of calcination.
Two stages of calcination are described by Butts in U.S. Pat. No. 2,442,969. Phosphate rock is mixed in specified proportion with phosphoric acid and then calcined at 1100.degree. to 1200.degree. C. This calcined intermediate is then ground to minus 50 mesh and mixed with a basic material such as lime to obtain a specified mole ratio and then calcined a second time at about 1000.degree. C. Butt suggests that about 20% of water may be added to the intermediate calcine-lime mix to facilitate the second calcining operation. However, grinding the intermediate and mixing with the basic material requires that the intermediate product be cooled to near ambient temperature. Since the material is calcined twice with intermediate cooling, unit energy consumption by this process is approximately doubled compared to single stage operation.
Hollingsworth, U.S. Pat. No. 2,562,718 also describes a two stage process in which a mixture of phosphoric acid and phosphate rock is calcined and then ground and mixed with lime and calcined a second time at a higher temperature. This two stage calcination also has approximately twice the energy consumption of a single stage calcination due to cooling the intermediate product to grind and mix with lime. In a subsequent disclosure, U.S. Pat. No. 2,556,541, Hollingsworth describes addition of lime to the material while it is in the kiln to enable calcination in a single pass or, alternately, in two or more passes through a shorter kiln with increasing hot zone temperatures in the succeeding passes.
Hollingsworth suggests that two stages of fluid bed calcination be used in U.S. Pat. No. 3,364,008. However, the preferable temperature in the first stage is about 1300.degree. F., below the minimum stated by Hollingsworth to be required for defluorination. Thus, the first bed agglomerates rather than defluorinates.